This invention relates generally to solar and other heat collectors and to methods of using such collectors and relates more particularly to solar collectors employing heat pipes in a gravity-assist mode and to methods of using such collectors.
Heat pipes are well known to be very efficient transmitters of heat. See, for example, the discussion by G. Yale Eastman, "The Heat Pipe," Scientific American, 218(5), pp. 38-46 (1968). As disclosed therein, the heat pipe is essentially a closed, evacuated chamber, the inside walls of which are lined with a capillary structure or wick that is saturated with a volatile fluid. In a typical cylindrical heat pipe, the input of heat to one end of the pipe causes the working fluid to evaporate from the wick and also increases the vapor pressure at the heat input end. As a result, the vapor moves down the core of the pipe, carrying heat energy toward the output end. When heat is removed from the pipe, the vapor condenses and the condensate goes back into the wick, returning to the input end by capillary action.
Generally, heat pipes are evacuated at least partially. As discussed in S. Katzoff, "Heat Pipes and Vapor Chambers for Thermal Control of Spacecraft," AIAA Paper 71-411, 1971, at page 14, generally in the prior art if the heat pipe were not at least partially evacuated (and a large percentage of noncondensable gas were present in the heat pipe along with the vapor), the noncondensable gas would be transported to the cold end of the heat pipe in operation, and would effectively blank off the condenser area, interfering with the operation of the heat pipe. However, not all heat pipes are completely evacuated. Sometimes controlled amounts of noncondensable gas are put into the heat pipe in order to provide for temperature regulation. See, for example, the discussion in "Heat Pipes for Temperature Control," by W. Bienert at the 4th Intersociety Energy Conversion Conference, Washington, DC, Sept. 1969. In general the amount of noncondensable gas remaining in the heat pipe must be carefully controlled to provide a range of operating conditions without completely stopping the heat transfer operation of the device. The determination of allowable quantities of noncondensable gas has been the subject of extensive analysis. See, for example, D. K. Edwards et al., "User's Manual for the TRW Gaspipe Program," TRW Document No. 13111-6022-RO-00, April 1971, pp. 2-1 to 2-5.
Heat pipes have been used in solar collectors in various configurations, both as high thermal conductance elements attached to plate-type collectors and as extended area vapor chambers. Both plate and chamber configurations have been operated in a gravity-assist mode. A vapor chamber, extended area heat pipe solar collector is described in J. C. Francken, "The Heat Pipe Fin, A Novel Design of A Planar Collector," 1975 International Solar Energy Congress and Exposition, Proceedings, HP 77-22025, which discloses a finned type of collector, based on the heat pipe principle. The design of that collector requires, however, that the heat pipe be evacuated in order for the apparatus to operate, and, consequently, that the heat pipe envelope be hermetically sealed.
In U.S. Pat. No. 4,067,315 to Fehlner et al., a solar collector heat pipe is disclosed which is an improvement on a known type of heat pipe adapted for use in the collecting of solar energy, in which a hermetically sealed chamber or envelope of tubular form has evacuated therefrom all noncondensable gases. However, the improvement of that invention also uses a hermetically sealed envelope; and the presence of noncondensable gas within the envelope does not at all appear to be desirable.
In U.S. Pat. No. 4,027,653 to Meckler, "Solar Energy Collector," one embodiment of the device of that invention (shown in FIGS. 4 and 6 in that patent) employs an interior region which is not evacuated. However, upon close examination of that patent, it appears clear that the description in the patent does not lead one to the heat collector of this invention, which operates quite differently from the devices in the patent. Using the design of the apparatus of FIGS. 4 and 6 of the patent, one would find that the noncondensable gas present in the unevacuated chamber would accumulate at the cool end of the tube and would limit the heat transfer to the circulated liquid to the latent heat of that vapor which diffused through the gas.
Therefore, despite what has been known in the prior art, a need has existed until now for a heat pipe solar collector having a relatively good efficiency, being easily assembled on-site and having a low cost.